FIG. 1 shows a conventional aircraft 1 having wings 2 attached to a fuselage 3, and engines in the form of a pair of gas turbine engines 4 mounted to the wings 2.
The aircraft 1 includes an environmental control system (ECS) 5. The ECS 5 provides pressurised, temperature controlled air to the aircraft cabin to aid passenger comfort, particularly at high altitude.
The ECS system 5 is a “bleedless” system, in which air is provided from a separate ECS compressor. The ECS compressor could be powered by an electrical motor for example, which is in turn powered by electricity generated by engine driven electrical generators. Alternatively, the ECS compressor could be driven by a separate prime mover, such as an Auxiliary Power Unit (APU) of the aircraft, or a dedicated prime mover. The pressure, flow rate and temperature of the air from the ECS compressor air is controlled using an air-cycle machine, and by cooling the compressed air with ambient air via a heat exchanger. This air is then delivered to the cabin, before being exhausted overboard once used through an exhaust port into the ambient airstream. The ambient air for the heat exchanger and the inlet air for the ECS compressor are drawn in from an intake duct 7.
Conventional ambient air intake ducts for ECS systems are located at the wing root of the aircraft where the fuselage 3 and leading edge of the wings 2 meet. This location has a high stagnation pressure, which results in a high pressure head being available to drive the ambient air through the heat exchanger and compressor inlet. U.S. Pat. No. 7,624,944 and EP1916185 each describe prior intake systems, in which high velocity air flow is separated from lower velocity air in the boundary layer adjacent the aircraft external wetted surface, with the higher velocity air being directed in to the ECS inlet. Such arrangements may increase the performance of the ECS, by increasing the pressure head to the compressor/heat exchanger inlet. Consequently, such an arrangement may reduce fuel burn in an aircraft, by reducing the energy needed to drive the compressor and/or increasing the airflow through the heat exchanger.
ECS systems which instead ingest a portion of the boundary layer close to the engine nacelle or wing have also been suggested in, for example, GB2247510. Such a design may prevent transition of laminar flow air to turbulent flow on the wing or engine nacelle In this case however, the ingested boundary layer air is used inefficiently, providing limited benefits which may not overcome the additional weight and/or complexity of the system.
An alternative method for reducing the fuel burn of an aircraft has been suggested, comprising ingesting air into the main aircraft propulsive engines, then exhausting this at the rear of the aircraft to fill in the wake from the aircraft, thereby reducing drag (see for example “Performance of a Boundary Layer Ingesting (BLI) Propulsion System”, published at the 45th AIAA Aerospace Sciences Meeting and Exhibit, 8-11 Jan. 2007, Reno, Nev.). However, such boundary layer ingestion systems may result in an inlet flow distortion penalty on main propulsive engine turbomachinery efficiency (with increased impact if the thrust generated is a large portion of aircraft thrust) which often offsets a large portion of the benefits of BLI.
In the art, the term “boundary layer” is a layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant. The thickness of the boundary layer is normally defined as the distance from the solid body at which the viscous flow velocity is 99% of the freestream velocity.
The present invention describes an aircraft air duct arrangement which results in increased aircraft performance.